One of the most important classes of indicators are the biological indicators (BI). Biological indicators provide the highest degree of assurance that sterilization conditions were met within the processor or processed load itself. This type of indicator is meant to represent the worst case for the processing system by providing an extremely high number of highly resistant organisms to that particular process within or on the indicator. Usually bacterial spores are the organism of choice for monitoring sterilization systems.
Biological indicators typically consist of microorganisms inoculated onto a carrier material. The microorganisms are typically bacterial spores that are known to be very resistant to the particular sterilization medium in which they are to be used. The carrier is placed into a sterilization cycle along with the medical device load. Following completion of the cycle the biological indicator is incubated and monitored for growth for up to seven days. Growth of a biological indicator indicates that the sterilization process was not adequate to attain complete sterilization and that the medical device load needs to be reprocessed before use. No growth of a biological indicator confirms that conditions within the sterilizer were adequate to kill at least the number of bacterial spores loaded onto the indicator (e.g., 10 bacterial spores) and therefore provides a level of assurance that the medical device load is sterile. Unfortunately many medical devices are actually used prior to the user knowing the results of the full incubation. Thus, there is a need in the hospital setting for detection of viable biological indicator spores in the shortest possible timeframe.
Historically, the detection of viable biological indicators relied on visual means of detection. The growth and multiplication of viable organisms can be seen/detected as evidenced by turbidity in the growth media. This turbidity can take days to become noticeable. Another visual and more common means of detection is with a colorimetric pH indicator. As viable organisms begin to metabolize and use up the nutrient sources such as sugars that are provided in the growth media, they excrete acidic waste products. As these acidic waste products accumulate in the growth media, the pH of the system is lowered resulting in a color change of the growth media if a pH indicator is present. Detection by this means usually takes 18-48 hours.
More recently, fluorescence has been used to detect the activity of enzymes that are produced by the organisms of interest by adding a fluorogenic enzymatic substrate to the growth media. This newer methodology lessens the incubation time from days to hours. However, the main limitation for reducing the incubation time beyond that seen for the fluorescence methodology is the inherent background fluorescence that naturally occurs with many components of the biological indicator including the plastic vials and growth media. Authentic, detectable signals must be high enough to be distinguishable over this inherent native background fluorescence. Therefore to increase the sensitivity of the system one needs to either reduce the background fluorescence (noise) or move to a different technology that has higher sensitivity (signal).
Thus, in the prior and current art, biological indicators rely on colorimetric or fluorometric means to determine viability. Detection is limited by the need for the generated signals, whether colorimetric or fluorometric, to be above substantial background levels. This has resulted in detection times for viable organisms on the order of hours to days in order for sufficient signal to be accumulated to be detectable above background levels. It would be beneficial for both hospitals and patients for the detection time of viable organisms in biological indicators to be on the order of minutes or less.